a) Field of the invention
The present invention, as can be best viewed in FIG. 1A, relates to a triplet lens system 1 which is suited for use with lens shutter cameras 2, etc., and has a stop 3 disposed on the image side of the lens system.
b) Description of the prior art
A large number of lens shutter cameras have conventionally adopted triplet lens systems owing to a fact that each of these lens systems can have favorable optical performance by using a small number of lens components and can easily be configured compact by.
Most of the conventional examples of the triplet lens systems belong to lens systems which have stops disposed after the lens systems since these lens systems are compatible with lens barrels having simple structures and advantageous not only for moving lens components, but also for exposure control.
For the triplet lens systems, each of which is composed of a small number of lens components and has low design freedom, however, there is a certain limit in configuring a photographic lens system which is composed only of spherical lens components, and has a wide field angle, a large aperture, a compact size and high optical performance. In particular, the triplet lens system which is composed only of the spherical lens components allows the optical performance thereof to be remarkably degraded, or produces coma flare and aggravates spherical aberration at the same time at intermediate and marginal angles within a range of a field angle thereof.
As conventional examples of the triplet lens systems which use aspherical lens components for solving these problems, there are known the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 59-34,510, Kokai Publication No. Sho 59-160,119, Kokai Publication No. Sho 59-160,120 and Kokai Publication No. Sho 63-96,620.
The triplet lens systems disclosed by these patents have relatively large apertures, wide field angles, compact compositions and favorable optical performance which are obtained by using the aspherical lens components. However, these photographic lens systems have F numbers on the order of 3.5 and cannot be said to be sufficient in brightness thereof.
Further, the triplet lens systems disclosed by Japanese Patents Kokai Publication No. Sho 59-160,119, Kokai Publication No. Sho 59-160,120 and Kokai Publication No. Sho 63-96,620, out of the above-mentioned conventional examples, select small departures from the reference sphere of the aspherical surfaces, thereby having insufficient functions to correct aberrations, or undercorrecting coma.
Furthermore, the triplet photographic lens systems disclosed by Japanese Patent Kokai Publication No. Sho 59-34,510 uses an aspherical surface on a first lens component thereof. However, such an aspherical surface used on the first lens component remarkably undercorrects spherical aberration though it has a function to correct coma, whereby the triplet lens system allows spherical aberration to be remarkably produced on the negative side.